Many states have adopted Renewable Portfolio Standards (RPS) that require electric utility companies to include a minimum amount of renewable energy generation as a percentage of the electric energy sold to their retail customers. The minimum RPS level will increase over time for most states. The federal government may soon implement a Renewable Electricity Standard (RES) that would be similar to the “renewables obligation” imposed in the United Kingdom. These standards place an obligation on electricity supply companies to produce a specified fraction of their electricity from renewable energy sources, such as wind, solar, hydroelectric, geothermal, biofuels, and biomass.
Further, the global initiative to reduce carbon emissions combined with the increasing cost and political uncertainty of fossil fuels has created a robust market for renewable energy. This market is accelerated by the regulations mentioned above, that require electric utilities to purchase a minimum amount of renewable energy, and by government subsidies (e.g., tax credits) for certain types of renewable energy technology. While some renewable energy facilities pay little to nothing for fuel, the low efficiency and high capital cost of the currently available power-generation technologies results in net costs that are significantly higher than those associated with traditional power generation techniques based on fossil fuels. For example, wind and solar energy have the widest geographic availability, but they also have low energy density.
More specifically, wind and solar power facilities must cover large geographic areas due to their low energy density to achieve the economies of scale required to justify the large construction investments, which increases the costs associated with connecting to the local power grid. Wind power generation has a lower capital cost per megawatt of installed capacity than solar power generation. Wind farms can also be installed around existing infrastructure (i.e. on buildings, above roads and farm fields, etc.). Together, these two conditions have resulted in the development of a large number of wind generation projects across the globe.
Wind turbines, however, have several technical constraints that affect performance. For example, 7-10 mph is typically the minimum wind speed required to turn a wind turbine fast enough to generate any usable electrical power. Further, wind turbines do not generate full rated power until the wind reaches a predetermined design speed, which is typically about 25-35 mph. When the wind speed is greater than about 50 mph, turbines must be turned off to avoid over-speed damage. These design constraints, along with wind variability, result in a very low capacity factor, i.e. total energy, in megawatt-hours, generated per year divided by the product of rated capacity, in megawatts, and 8760 hours per year. Wind generation facilities typically have a capacity factor of 25-35%. A solar farm may have a capacity factor of 40-45%, coal burning power plants have a capacity factor of 60-80%, and nuclear power plants have a capacity factor of 70-90%.
Electric utilities are tasked with providing reliable electric service to all customers, and so the dependence on large amounts of generation from often-variable and dynamic wind fluctuations is detrimental to power grid stability. Currently, to quickly address additional power load demands or balance load reductions, on-line generators are used to ramp up or down power generation to match a new load demand. Commonly, utilities operate a number of generators in parallel to provide base load capacity (typically the most efficient, running 24/7), cycling capacity (which cycle up and down with daily load fluctuations), and peaking capacity (to meet sudden short term or peak load demands).
When utilities are using intermittent generation sources, such as wind energy, fluctuations in the wind generation must also be balanced relative to the other utility generation sources. Thus a “spinning reserve” is needed that is a function of the energy-producing units that are already in operation. If the capacity of wind generation changes, up or down, more than what the other utility generators can absorb, this causes instability that must be absorbed across the entire grid. If a utility cannot balance all the up and down fluctuations from the connected wind generation in its service territory, the utility will not be able to schedule reliable electrical service to its customers without some deficiencies in power quality, such as voltage regulation, reactive load, and frequency regulation. Given that most utilities operate with only a 15% to 25% reserve capacity margin, which is primarily designed to cover scheduled and forced outages of their primary generating plants, the amount of flexible generating capacity for balancing energy becomes the limiting factor in the amount of wind generation that can be managed for a given utility.
The primary method used by utilities to convert fuel energy into electricity is to burn fuel (coal or gas, for example), recover the thermal energy as high pressure superheated steam, and use the steam to drive a steam turbine generator. Steam turbine generators (STG's) are used in coal plants, nuclear plants, gas fired boilers, waste-to-energy, biomass-to-energy, waste heat recovery, and in a portion of gas fired combined cycle plants. Starting the STG from a “cold iron” condition requires several hours because the temperature of the entire system, including the STG, must be slowly raised to avoid excessive thermal stresses which could damage the generator. If starting from “hot standby” mode, the STG may still take up to an hour to start generating power because the turbine spin rate must be slowly and carefully increased while passing through critical resonance speeds that can cause excessive vibrations and generator damage. Once operating, changing the power output of steam turbine generators is limited by the speed that additional feed water can be brought up to a boil to produce the additional steam required to turn the turbine.
While STG's can balance some of the fluctuations associated with wind speed variations and instability, there are conditions that require much faster and larger power output. Utilities typically use fast starting (nominally about 10 minutes) gas fired simple cycle combustion turbines to provide this type of response. By contrast, a combined cycle combustion turbine plant generates about 50% more electricity for the same fuel input. More specifically, a combined cycle system uses a heat recovery steam generator that draws heat from the exhaust of a simple cycle process to generate additional high pressure superheated steam to drive a STG. A simple cycle combustion turbine, however, avoids the delays associated with starting the steam system in a combined cycle plant. A simple cycle is penalized with a much higher heat rate (i.e., the rate of fuel consumption per kilowatt-hour [kWh] of generation) because a simple cycle system simply sends hot exhaust gases from the gas turbine directly to the atmosphere.
As more and more wind capacity is placed into service, the market requires more flexible capacity that can be dispatched on short notice to balance the up and down fluctuations of wind generation. With fossil fuel prices much higher than historical averages, it is also important that the additional capacity be generated as fuel efficiently and economically as possible. The market demand is for flexible spinning reserve capacity that can be economically used to balance wind generation.
Thus, it is a long felt and fundamental need in the field of wind power generation to provide a balancing system that addresses fluctuations and power generation instabilities. The following disclosure describes an improved energy balancer that works in conjunction with a wind power facility to ensure that the needed power is consistently generated. As requirements for renewable energy increase, utilities will need resources that have more flexibility than the traditional increments of base-load, cycling, and peaking capacity. Load management will require flexible spinning reserve as intermittent resources are added to the generation mix. This resource configuration, as described below, will be able to supplement all of the traditional utility resources in addition to providing the flexibility required to manage wind generation.